全文获取类型
收费全文 | 2127篇 |
免费 | 150篇 |
国内免费 | 128篇 |
专业分类
电工技术 | 142篇 |
综合类 | 162篇 |
化学工业 | 643篇 |
金属工艺 | 103篇 |
机械仪表 | 75篇 |
建筑科学 | 78篇 |
矿业工程 | 27篇 |
能源动力 | 129篇 |
轻工业 | 277篇 |
水利工程 | 12篇 |
石油天然气 | 68篇 |
武器工业 | 15篇 |
无线电 | 117篇 |
一般工业技术 | 245篇 |
冶金工业 | 86篇 |
原子能技术 | 21篇 |
自动化技术 | 205篇 |
出版年
2024年 | 7篇 |
2023年 | 48篇 |
2022年 | 64篇 |
2021年 | 74篇 |
2020年 | 71篇 |
2019年 | 52篇 |
2018年 | 75篇 |
2017年 | 78篇 |
2016年 | 80篇 |
2015年 | 85篇 |
2014年 | 116篇 |
2013年 | 130篇 |
2012年 | 122篇 |
2011年 | 158篇 |
2010年 | 128篇 |
2009年 | 114篇 |
2008年 | 101篇 |
2007年 | 133篇 |
2006年 | 136篇 |
2005年 | 93篇 |
2004年 | 81篇 |
2003年 | 77篇 |
2002年 | 46篇 |
2001年 | 46篇 |
2000年 | 41篇 |
1999年 | 43篇 |
1998年 | 28篇 |
1997年 | 27篇 |
1996年 | 16篇 |
1995年 | 16篇 |
1994年 | 20篇 |
1993年 | 14篇 |
1992年 | 11篇 |
1991年 | 19篇 |
1990年 | 16篇 |
1989年 | 7篇 |
1988年 | 5篇 |
1987年 | 4篇 |
1986年 | 5篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 1篇 |
1978年 | 1篇 |
1972年 | 1篇 |
1959年 | 1篇 |
排序方式: 共有2405条查询结果,搜索用时 15 毫秒
1.
《Ceramics International》2022,48(9):12281-12290
Following the rapid growth of lightning technology, the development of red-emitting phosphors is effective for improving color temperature and color rendering index for w-LEDs devices. Herein, a single phased garnet phosphor with cation and polyhedron substitution modification was firstly prepared. For Mg3Gd2Ge3O12: Bi3+, Eu3+, the intensity has been remarkably improved by about 16% compared to the one without Bi3+ sensitization. The energy transfer mechanism is identified in this work. Based on cation and polyhedron substitution strategies, novel phosphors with different compositions were obtained and further modified the PL properties. With Lu3+ substitution, the bond lengths between Bi3+ ion and anion ligands are decreased and the site symmetry has been strengthened, which leads to a 21 nm blue shift when Lu3+ totally replaced Gd3+ ions. In addition, Lu3+ and [SiO4] substitution strategies both effectively increased symmetric rigid structure, which leads to a significant improvement in thermal stability, indicating the samples own great potential in optical applications This work provides a new insight to synthesis red-emitting phosphors for warm white-LEDs. 相似文献
2.
《Ceramics International》2022,48(9):12537-12548
Enhancing the electrical conductivity of electrode materials via a cationic substitution strategy was recognized as an effective way of improving the electrochemical performance of Li-ion batteries. Thus, LixCa1-xFe2O4 nanoparticles were synthesized via a facile inexpensive process at low temperature. XRD peaks refer to the formation of an orthorhombic structure with the Pnma space group. HR-TEM investigations reveal orthorhombic-like shape for pure CaFe2O4, nanoplatelet-like morphology for Li0.05Ca0.95Fe2O4 and irregular distorted crystals for Li0.1Ca0.9Fe2O4. Voids and pores in Li-doped CaFe2O4 were confirmed by FESEM and BET measurements. XPS spectra of O1s prove that Li-doped CaFe2O4 have higher conductivity due to the created lattice defects and oxygen species. Li-doped CaFe2O4 anodes exhibit great improvement in their initial discharge capacities ~1219 and 1606 mAhg?1 upon substitution of Ca with 5% and 10% Li, respectively. Furthermore, 10% Li-doped CaFe2O4 anode displays the highest Li-ions diffusion coefficient and exchange current density due to the enhanced Li+ ions mobility. Moreover, the DC activation energies for the LixCa1-xFe2O4 nanoparticles decreased with increasing Li content. 相似文献
3.
《Ceramics International》2022,48(17):24859-24865
Ca3Co4O9+δ is a typical p-type thermoelectric oxide material with a low thermal conductivity. In this study, double-layered oxide samples Ca(Ba,Sr)3Co4O9+δ dispersed with different SiC contents were obtained via the traditional solid phase reaction method. The effects of different elemental substitutions and SiC dispersion contents on the microstructure and thermoelectric properties of the samples were studied. The double optimisation of partial substitution of Ca-site atoms and SiC dispersion considerably improved the thermoelectric properties of Ca3Co4O9+δ. Through the elemental substitution, the resistivity of the Ca3Co4O9+δ material was reduced. Conversely, introducing an appropriate amount of SiC nanoparticles enhanced phonon scattering and was crucial in reducing its thermal conductivity. After double optimisations, the dimensionless thermoelectric figure of merit (ZT) values of both Ca2.93Sr0.07Co4O9+δ + 0.1 wt% SiC and Ca2.9Ba0.1Co4O9+δ + 0.1 wt% SiC achieved an optimum value of 0.25 at 923 K. 相似文献
4.
To investigate the evolution of the structural and enhanced magnetic properties of GdMnO3 systems induced by the substitution of Mn with Cr, polycrystalline GdMn1-xCrxO3 samples were synthesized via solid-state reactions. XRD characterization shows that all GdMn1-xCrxO3 compounds with single-phase structures crystallize well and that Cr3+ ions entering the lattice sites of GdMnO3 induce structural distortion. SEM results indicate that the grain size of the synthesized samples (a few microns) decreases as the Cr substitution concentration increases. Positron annihilation lifetime spectroscopy reveals that vacancy-type defects occur in GdMn1-xCrxO3 ceramics and that the vacancy size and concentration clearly change with the Cr content. The temperature and field dependence of the magnetization curves show that Cr substitution significantly influences the magnetic ordering of the gadolinium sublattice, improving the weak ferromagnetic transition temperature and magnetization of GdMn1-xCrxO3. The enhanced magnetization of GdMn1-xCrxO3 is closely related to the vacancy defect concentration. 相似文献
5.
Traditional optimization methods, which take a specific order of modal frequency as the design constraint, could fail to obtain the desired solution because of modal substitution. An improved optimization model with continuous sizing variables is established to solve this problem, in which the minimum weight and a given local modal frequency are considered as the objective and the constraint. To capture accurately the expected mode of vibration, a local mode identification technique is proposed based on the strain energy ratio between the local area and the whole structure. With that scheme, an optimization system is developed, in which the local mode can be effectively identified and the constraint can be updated with it in the iteration process. Two numerical examples, of a reinforced plate and a satellite structure, are applied to illustrate the effectiveness and efficacy of the proposed method. 相似文献
6.
Jiangguo Xiao Wentao Zhang Teshen Wang Jiyong Zhang Haiying Du 《Ceramics International》2021,47(9):12028-12037
A series of Na5Y(MoO4)4-yAy:Dy3+ (A = WO42?, VO43?; y = 0–0.05) phosphors were synthesized by the combustion method. Some of the MoO42? sites were occupied by WO42? and VO43? anions, which enhanced the luminescence property of Dy3+-doped Na5Y(MoO4)4. XRD results show that the crystal structures of the samples were consistent with the standard Na5Y(MoO4)4 phase. Under excitation at 352 nm, the Na5Y(MoO4)4-yAy:Dy3+ phosphors exhibited a characteristic blue emission at 485 nm and a yellow emission at 577 nm, which originated from the 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 transitions of Dy3+ ions, respectively. White light can be achieved by combining these blue and yellow emissions. After replacing MoO42? with WO42? and VO43? anions in Na5Y(MoO4)4:Dy3+, the luminescence intensity of Dy3+ was significantly improved due to the crystal field effect. The results indicate that Na5Y(MoO4)3.97(WO4)0.03:Dy3+ and Na5Y(MoO4)3.97(VO4)0.03:Dy3+ phosphors have good prospects for application in n-UV-excited w-LEDs. 相似文献
7.
8.
Su-Il Ri Song-Il Hong Jong-Chol Kim Ju-Hyok Wi 《International Journal of Hydrogen Energy》2021,46(5):3319-3328
We investigated some properties of the hydride Mg2FeH6 substituted with yttrium by a first principles calculation. Some experimental results showed that 4d transition metal, yttrium serves as a good catalyst for magnesium based hydrogen storage alloys, but there are a few theoretical studies about magnesium based hydrides substituted with it. Mg2FeH6 is regarded as a cheaper material than pure MgH2, while it is crystalized into Fm3m structure (space group 225). Although it has high hydrogen storage capacity, many investigations have not been devoted to it due to its extremely high thermodynamic stability. The yttrium substituted Mg2FeH6 exhibits very low energy of formation, and its desorption temperature, 75 °C is very suitable for practical hydrogen storage applications. Our results showed that Mg2FeH6 is destabilized effectively by yttrium substitution and introducing vacancy defects has additive effect to the improvement of dehydrogenation performance. 相似文献
9.
Jaruneth Petchoo Sitima Jittinandana Siriporn Tuntipopipat Chitraporn Ngampeerapong Nattapol Tangsuphoom 《International Journal of Food Science & Technology》2021,56(4):1750-1758
This study determined the physicochemical, sensorial and nutritional properties of breadsticks to which a resistant starch (RS)-rich ingredient was incorporated by partial substitution, that is, 40%–70%, of hard wheat flour in the control recipe. Wheat flour substitution at the levels exceeding 50% lightened the colour while decreasing the moisture content and hardness of breadsticks, which was explainable by the microstructure. Changes in properties of breadsticks also affected the suitability of its sensory attributes, particularly colour and texture. Breadsticks, of which half of wheat flour was substituted, contained less protein and calories but higher carbohydrate than the control formula. Such flour substitution increased RS content closed to its effective dose of 15 g in a 45-g serving, which led to approximately 15% reduction in in vitro glycaemic index. Therefore, incorporating effective dose of RS in breadsticks could be performed, without causing substantial changes in product quality, by partial substitution of wheat flour. 相似文献
10.
Wenyun Yang Qiwei Hu Guanyi Qiao Liang Zha Shunquan Liu Jingzhi Han Honglin Du Yan Zhang Yingchang Yang Changsheng Wang 《中国稀土学报(英文版)》2019,37(10):1102-1107
In this work, we report the tuning effect of the Si substitution on the magnetic and high frequency electromagnetic properties of R2Fe17 compounds and their paraffin composites. It is found that the introduction of Si can remarkably improve the magnetic and electromagnetic properties of the R2Fe17 compounds, making the R2Fe17–xSix-paraffin composites excellent microwave absorption materials (MAMs). By introducing the Si element, their saturation magnetizations decrease slightly, while much higher Curie temperatures are obtained. Furthermore, better impedance match is reached due to the decrease of the high-frequency permittivity ε′ by about 40%–50%, which finally enhances the performance of the microwave absorption. The peak frequency (fRL) of the reflection loss (RL) curve moves toward high frequency domain and the qualified bandwidth (QB, RL ≤ ?10 dB) increases remarkably. The maximum QB of 3.3 GHz (12.0–15.3 GHz) is obtained for the Sm1.5Y0.5Fe15Si2-paraffin composite (d = 1.0 mm) and the maximum RL of ?53.6 dB is achieved for Nd2Fe15Si2-paraffin composite (d = 2.2 mm), both surpassing most of the reported MAMs. Additionally, a distinguished dielectric microwave absorption peak is observed, which further increases the QB in these composites. 相似文献